The term website quality assurance, when applied to websites, describes the process of enforcing quality control principles and functioning to advance the processes that are used in producing the website and its workings, communications and content. When website quality assurance is well implemented, a website quality should see progressive development in terms of both attenuation rate of defects and general increase in website usability and performance.
Quality assurance should function as a "tone" for the user, a reminder to the designers and developers that the site is designed for users outside the office. Quality assurance as ombudsman would be a positive force for a quality user experience. If you are limited in what you can accept responsibility for, document those limits. For example, if you can't test data or middleware, announce that fact whenever you provide test results for the website quality. Even the best designed and developed sites will experience problems and failures, so a good quality assurance team should set expectations -- for the entire web site team and with management -- for what QA can effectively accomplish.
Focus on Improving Processes
The key to understanding website quality assurance is, understanding the emphasis on process: quality control focuses on what comes out of the web site creation process (creation, development, publication -- whatever term you prefer that describes the process that results in the web site). Quality assurance focuses on what goes into the creation process as well as on the process itself with the goal of improving the quality of output by improving everything "downstream".
Quality assurance looks beyond the structured testcases used by website quality control because these testcases are necessarily limited. Quality assurance focuses on more than a site's ability to meet a specific benchmark; quality assurance aims to make the site better so tests are passed more consistently, so that the benchmark can in fact be refined, and so that problem areas can be eliminated.
Quality assurance should be involved in the development process. QA should review new designs before they are finalized with an eye towards usability and user experience factors; heading of problems before they become real improves quality immediately and reduces problems "downstream”. Quality assurance should be involved in customer service and user-support communications, especially with a commerce site, so that usability defects can be reviewed. With user input, QA can refine user scenarios to better match "real" behavior. There is no substitute for user comments.
Following any major phase of your website, perform a postmortem analysis: review the success of the changes, redesigns, scheduling, file transfers, etc. What could be made more efficient? Which processes could be smoothed out?